Most planar molecular are carcinogenic because they are able to wedge themselves between base pairs on DNA (by virtue of being flat). I would guess that it's the same with benzene.

Why is it that 9-coordinate inorganic molecules like the tri-capped trigonal prism ReH<sub>9</sub><sup>2-</sup> are only found with d<sup>0</sup> metals? In other words, why do 9-coordinate molecules only occur with transition metals that have been oxidized until they have lost all d electrons)?

If air at 25 degrees C is compressed in a heat-exchanger where water is at 25 degrees C. The air chamber of the exchanger is sealed and the pressure is increased up to 450 atm by adding more air. After all heat is exchanged, the pressurized air is expanded back to one atmosphere. What is the temperature of the expanded air?

If air at 25 degrees C is compressed in a heat-exchanger where water is at 25 degrees C. The air chamber of the exchanger is sealed and the pressure is increased up to 450 atm by adding more air. After all heat is exchanged, the pressurized air is expanded back to one atmosphere. What is the temperature of the expanded air?

Probably need to look up some numbers :-)

Maybe I'm missing something, but if both the air and the water are 25 C, there shouldn't be any heat exchange. Everything will still be 25 C, right?

Even if you compress the air its temperature will rise. Im rubbish at mathematics so cant handle equations but it goes something like this.....

.....The denser the air is due to compression the more molecules per square (unit of cubic measurment) so that will mean that they will bump into each other more frequently and will cause the temperature to rise.

Next you use a hand pump to pump up your bicycle's tyre....feel how hot the metal parts of the pump get !!

The hand of time rested on the half-hour mark, and all along that old front line of the English there came a whistling and a crying. The men of the first wave climbed up the parapets, in tumult, darkness, and the presence of death, and having done with all pleasant things, advanced across No Man's Land to begin the Battle of the Somme. - Poet John Masefield.

A giveaway would be that the pseudo-Joule-Thompson coefficient of air at 1 atm and 273K (forgive me when I wrote the question but its close enough that it should not matter) is approximately 2.26 K/MPa.

If you take air at 25c and compress it it's temperature will rise - see 'Charles and Gay-Lussac's Law '

I'm pretty sure that the Charles/Gay-Lussac gas laws deal with the relationship between temperature and pressure in systems with a fixed volume and a fixed number of mols of gas. If you pressurize it by adding more air (as stated in the question) you are no longer dealing with a Charles/Gay-Lussac system. If you increase the pressure by simply adding more gas, I don't know of any law saying that the temperature will change.

Also (and it's been a while since my last real pchem class, so I might be wrong about this) I thought that the Joule-Thompson effect was only for gases with a fixed number of mols that are allowed to exand at a constant enthalpy.

A refridgerator works by absorbing and releasing the heat of vaporization of a liquid that has a low boiling point, like freon or amonia.

You allow a liquid like freon to evaporate when it's in tubes inside the refridgerator, which cools down the inside because the freon (or whatever) is absorbing its heat of vaporization. You then pump the gas outside the cooling chamber and re-condense it with a compressor; the gas releases its heat of vaporization, but now you're outside the cooling chamber, so it doesn't matter. The inside cools down and the rest of the room heats up.

But there didn't seem to be any phase changes involved in the original question.

Well it's a volume change accompanied by a temperature change, whether it goes through a phase change is immaterial - it is essentially the same process, compress the volume of a fixed mass and it's temp rises - It even workd on the stellar scale.

Temperature Rise Ratio - The ratio of the computed isentropic temperature rise to the measured total temperature rise during compression. For a perfect gas, this is equal to the ratio of the isentropic enthalpy rise to the actual enthalpy rise.